Takaaki Kajita

Nobel Prize Laureate Takaaki Kajita

Resolving the Neutrino Puzzle

The 2015 Nobel Prize in Physics was awarded jointly to Japanese scientist Dr. Takaaki Kajita and Canadian scientist Arthur B. McDonald for the “discovery of neutrino oscillations, which shows that neutrinos have mass.” Modern physics uses the Standard Model, which defines three differerent types, or “flavors,” of a very small, elusive particle called the neutrino. In 1998, Dr. Takaaki Kajita detected neutrinos that were created in reactions between cosmic rays and the Earth’s atmosphere inside the Super-Kamiokande detector, an experimental facility in a Japanese mine. Measurements showed deviations, which were explained by the neutrinos switching between the differerent “flavors.” This is ultimately meant that neutrinos must have mass. As the Standard Model is based on the theory that neutrinos lack mass, this research meant that the model must be revised. Dr. Kajita was born in 1959. In 1981, he started his scientific career in the graduate program at the University of Tokyo, where he received his Ph.D. in physics in 1986. After graduating, he began working at the University of Tokyo’s International Center for Elementary Particle Physics. In 1988, he moved to the University of Tokyo’s Institute for Cosmic Ray Research and has served as its director since 2008. Tokyo Journal Executive Editor Anthony Al-Jamie talked to Dr. Takaaki Kajita about his contributions to the eld of physics.

TJ: How did you first get interested in physics?KAJITA: When I was a high school student, I studied physics, and during my undergraduate studies, I thought physics was interesting so I decided to do research in my graduate program. During my graduate program, I joined Professor [Masatoshi] Koshiba’s group and the Kamiokande experiment. That was a truly fantastic experience for me, so I decided to work in the field of physics.

TJ: Did you struggle with anything during your doctoral studies?KAJITA: I was struggling with physics in English. At that time, I was able to write about five lines in English, but for the thesis we had to write more than 100 pages. That was a big problem for me [laughs].

TJ: Is there someone who had a big influence on you?KAJITA: My supervisor, Professor Koshiba, had a big influence on me. Professor Koshiba was the spokesperson for Kamiokande and I received my Ph.D. for the analysis of the Kamiokande data. He told me that experimental research is important for contributing to the scientific field. From day-to-day research, I really found that what he told us is true, so I enjoy experiments.

TJ: Can you tell us about the project that resulted in you receiving the Nobel Prize?KAJITA: The main project that led me to receive the Nobel Prize is the Super- Kamiokande experiment, which was very important to the discovery of neutrino mass. Super-Kamiokande was an extremely good detector that was able to convince the scientific community that neutrinos oscillate. Before Super-Kamiokande, we had been working on the Kamiokande experiment on atmospheric neutrinos. For almost 10 years we had been finding some puzzling data on atmospheric neutrinos — that is the deficit of muon neutrinos. I think the studies in Kamiokande were really important to let the community know that maybe neutrinos do oscillate, and as soon as the results of Super-Kamiokande demonstrated the discovery of neutrino oscillation, the community was convinced that the research carried out in Kamiokande was very important.

TJ: Did you expect to receive the Nobel Prize?KAJITA: Well, no [laughs]. When I presented the Super-Kamiokande results in 1998, I didn’t expect to receive the Nobel Prize. But in recent years, every October, the Japanese media tried to interview me about it at the University of Tokyo. I realized that there could be a possibility, but not a strong one.

TJ: How did you feel when it was announced that you were going to be receiving the award?KAJITA: Actually, I was shocked! I was unable to think about anything for a while. I feel that this is a real honor for me.

TJ: Why is this information important to society?KAJITA: In the 20th century, the Standard Model of particle physics was established. However, people also realized that the Standard Model is not the ultimate theory of particle physics. So people wanted to have some new hint towards a better understanding of particle physics and one of the first hints was neutrino mass. Therefore, I think neutrino mass is extremely important for particle physicists to think about for the next step in particle physics.

TJ: How many years did this research take?KAJITA: I got serious about the atmospheric neutrinos deficit in 1986, and in 1998, the Super-Kamiokande experiment proved that neutrinos oscillate. However, even after that we have been researching further details of neutrino oscillation, so I think I spent almost 20 years studying this.

TJ: I understand the Kamiokande research facility is under Mount Ikeno. How much time did you spend working underground?KAJITA: In the Japanese fiscal year 1995, we were at the detector construction stage and basically, I spent a full year underground for the construction of the Super-Kamiokande detector. Even before that, I spent a lot of time underground for the improvement or modification of the Kamiokande detector.

TJ: Who was the first person that you contacted after receiving notice that you were going to be granted this award?KAJITA: I think I first contacted Professor Koshiba and then my wife. Then the third person was the president of the University of Tokyo. He was very happy!

TJ: How about Dr. Koshiba? What did he say to you?KAJITA: I think he was happy. I think he just said congratulations.

TJ: We noticed in your acceptance speech that you thanked your wife. In what ways was your wife supportive of you in this research?KAJITA: Well, basically, I really concentrated on my research and I didn’t take care of my family — my children. So she did everything for our family.

TJ: Does that mean she gets half of the prize money? [Laughs.] What will you do with the prize money?KAJITA: [Laughs.] Super Kamiokande was supported very strongly by the Japanese cosmic ray community. So I would like to use this money for our institute for cosmic ray research. That is what I am thinking of doing, but I have not decided yet.

TJ: I understand you’ve received other awards in addition to the Nobel Prize for this discovery. Which was the most special?KAJITA: Yes, several. In the Japanese community, there is an award called the Nishina Memorial Prize. This is a very prestigious prize for Japanese physicists and I was very happy to receive this prize in 1999.

TJ: Is being able to understand, read and write in English important in your work?KAJITA: Yes, I think so. English is definitely the language for science, so without knowing the language we are unable to communicate. I think that the English level of Japanese scientists is not as good as other non-English-speaking countries. I don’t know why, but this is a problem we have to face.

TJ: Do you have a message for young scientists just entering the field?KAJITA: I want to say that from my experience, young people have a greater chance of discovering something new because they are the ones who are actually looking at the data.

TJ: What are your dreams for the future?KAJITA: My dream is simple: to continue researching. tj

The complete article can be found in Issue #278 of the Tokyo Journal. Click here to order from Amazon.